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Merge pull request #5159 from maron2000/fix_imgmake_bpb
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Fix incorrect BPB of floppy images made by IMGMAKE command
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@joncampbell123
joncampbell123 authored Aug 9, 2024
2 parents 0cefb8d + 4462d88 commit ca2d626
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Showing 2 changed files with 97 additions and 27 deletions.
28 changes: 28 additions & 0 deletions include/bios_disk.h
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Original file line number Diff line number Diff line change
Expand Up @@ -42,6 +42,34 @@ extern diskGeo DiskGeometryList[];

extern const uint8_t freedos_mbr[];

#define BytsPerSec 0x0b /* 2-bytes */
#define SecPerClus 0x0d /* 1-byte */
#define RsvdSecCnt 0x0e /* 2-bytes */
#define NumFATs 0x10 /* 1-byte */
#define RootEntCnt 0x11 /* 2-bytes */
#define TotSec16 0x13 /* 2-bytes */
#define Media 0x15 /* 1-byte */
#define FATSz16 0x16 /* 2-bytes */
#define SecPerTrk 0x18 /* 2-bytes */
#define NumHeads 0x1a /* 2-bytes */
#define HiddSec 0x1c /* 4-bytes */
#define TotSec32 0x20 /* 4-bytes */
#define DrvNum 0x24 /* 1-byte */
#define BootSig 0x26 /* 1-byte */
#define VolID 0x27 /* 4-bytes */
#define VolLab 0x2b /* 11-bytes*/
#define FilSysType 0x36 /* 8-bytes */
#define BootCode 0x3e
#define BootSign 0x1fe /*2-bytes */

#define FATSz32 0x24 /* 4-bytes */
#define DrvNum32 0x40 /* 1-byte */
#define BootSig32 0x42 /* 1-bytes */
#define VolID32 0x43 /* 4-bytes */
#define VolLab32 0x47 /* 11-bytes*/
#define FilSysType32 0x52 /* 8-bytes */
#define BootCode32 0x5A

class imageDisk {
public:
enum IMAGE_TYPE {
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96 changes: 69 additions & 27 deletions src/dos/dos_programs.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -3639,26 +3639,38 @@ class IMGMAKE : public Program {

uint8_t mediadesc = 0xF8; // media descriptor byte; also used to differ fd and hd
uint16_t root_ent = 512; // FAT root directory entries: 512 is for harddisks
uint16_t disksize = 0;
bool is_fd = false;
if(disktype=="fd_160") {
c = 40; h = 1; s = 8; mediadesc = 0xFE; root_ent = 56; // root_ent?
disksize = 160; is_fd = true;
} else if(disktype=="fd_180") {
c = 40; h = 1; s = 9; mediadesc = 0xFC; root_ent = 56; // root_ent?
disksize = 180; is_fd = true;
} else if(disktype=="fd_200") {
c = 40; h = 1; s = 10; mediadesc = 0xFC; root_ent = 56; // root_ent?
disksize = 200; is_fd = true;
} else if(disktype=="fd_320") {
c = 40; h = 2; s = 8; mediadesc = 0xFF; root_ent = 112; // root_ent?
disksize = 320; is_fd = true;
} else if(disktype=="fd_360") {
c = 40; h = 2; s = 9; mediadesc = 0xFD; root_ent = 112;
disksize = 360; is_fd = true;
} else if(disktype=="fd_400") {
c = 40; h = 2; s = 10; mediadesc = 0xFD; root_ent = 112; // root_ent?
disksize = 400; is_fd = true;
} else if(disktype=="fd_720") {
c = 80; h = 2; s = 9; mediadesc = 0xF9; root_ent = 112;
disksize = 720; is_fd = true;
} else if(disktype=="fd_1200") {
c = 80; h = 2; s = 15; mediadesc = 0xF9; root_ent = 224;
disksize = 1200; is_fd = true;
} else if(disktype=="fd_1440"||disktype=="fd"||disktype=="floppy") {
c = 80; h = 2; s = 18; mediadesc = 0xF0; root_ent = 224;
disksize = 1440; is_fd = true;
} else if(disktype=="fd_2880") {
c = 80; h = 2; s = 36; mediadesc = 0xF0; root_ent = 512; // root_ent?
disksize = 2880; is_fd = true;
} else if(disktype=="hd_250") {
c = 489; h = 16; s = 63;
} else if(disktype=="hd_520") {
Expand Down Expand Up @@ -4090,15 +4102,15 @@ class IMGMAKE : public Program {
// reserved sectors
host_writew(&sbuf[0x0e],reserved_sectors);
// Number of FATs
sbuf[0x10] = fat_copies;
sbuf[NumFATs] = fat_copies;
// Root entries if not FAT32
if (FAT < 32) host_writew(&sbuf[0x11],root_ent);
if (FAT < 32) host_writew(&sbuf[RootEntCnt],root_ent);
// sectors (under 32MB) if not FAT32 and less than 65536
if (FAT < 32 && vol_sectors < 65536ul) host_writew(&sbuf[0x13],vol_sectors);
if (FAT < 32 && vol_sectors < 65536ul) host_writew(&sbuf[TotSec16],vol_sectors);
// sectors (32MB or larger or FAT32)
if (FAT >= 32 || vol_sectors >= 65536ul) host_writed(&sbuf[0x20],vol_sectors);
// media descriptor
sbuf[0x15]=mediadesc;
sbuf[Media]=mediadesc;
// sectors per FAT
// needed entries: (sectors per cluster)
Bitu sect_per_fat=0;
Expand Down Expand Up @@ -4145,60 +4157,90 @@ class IMGMAKE : public Program {
}

// sectors per track
host_writew(&sbuf[0x18],s);
host_writew(&sbuf[SecPerTrk],s);
// heads
host_writew(&sbuf[0x1a],h);
host_writew(&sbuf[NumHeads],h);
// hidden sectors
host_writed(&sbuf[0x1c],(uint32_t)bootsect_pos);
host_writed(&sbuf[HiddSec],(uint32_t)bootsect_pos);
/* after 0x24, FAT12/FAT16 and FAT32 diverge in structure */
if (FAT >= 32) {
host_writed(&sbuf[0x24],(uint32_t)sect_per_fat);
host_writed(&sbuf[FATSz32],(uint32_t)sect_per_fat);
sbuf[0x28] = 0x00; // FAT is mirrored at runtime because that is what DOSBox-X's FAT driver does
host_writew(&sbuf[0x2A],0x0000); // FAT32 version 0.0
host_writed(&sbuf[0x2C],2); // root directory starting cluster
host_writew(&sbuf[0x30],1); // sector number in reserved area of FSINFO structure
host_writew(&sbuf[0x32],6); // sector number in reserved area of backup boot sector
// BIOS drive
if(mediadesc == 0xF8) sbuf[0x40]=0x80;
else sbuf[0x40]=0x00;
sbuf[DrvNum32] = (mediadesc == 0xF8) ? 0x80 : 0x00;
// ext. boot signature
sbuf[0x42]=0x29;
sbuf[BootSig32] = 0x29;
// Volume label
sprintf((char*)&sbuf[VolLab32], "NO NAME ");
// volume serial number
// let's use the BIOS time (cheap, huh?)
host_writed(&sbuf[0x43],mem_readd(BIOS_TIMER));
// Volume label
sprintf((char*)&sbuf[0x47],"NO NAME ");
host_writed(&sbuf[VolID32], mem_readd(BIOS_TIMER));
// file system type
sprintf((char*)&sbuf[0x52],"FAT32 ");
sprintf((char*)&sbuf[FilSysType32],"FAT32 ");
}
else { /* FAT12/FAT16 */
// BIOS drive
if(mediadesc == 0xF8) sbuf[0x24]=0x80;
else sbuf[0x24]=0x00;
// ext. boot signature
sbuf[0x26]=0x29;
sbuf[BootSig] = 0x29;
// Volume label
sprintf((char*)&sbuf[VolLab], "NO NAME ");
// volume serial number
// let's use the BIOS time (cheap, huh?)
host_writed(&sbuf[0x27],mem_readd(BIOS_TIMER));
// Volume label
sprintf((char*)&sbuf[0x2b],"NO NAME ");
host_writed(&sbuf[VolID], mem_readd(BIOS_TIMER));
if(is_fd) {
uint8_t index = 0;
while(DiskGeometryList[index].cylcount != 0) {
if(DiskGeometryList[index].ksize == disksize) {
sbuf[Media] = DiskGeometryList[index].mediaid;
host_writew(&sbuf[SecPerTrk],DiskGeometryList[index].secttrack);
host_writew(&sbuf[NumHeads], DiskGeometryList[index].headscyl);
host_writew(&sbuf[BytsPerSec], DiskGeometryList[index].bytespersect);
sbuf[SecPerClus] = DiskGeometryList[index].sectcluster;
host_writew(&sbuf[RootEntCnt], DiskGeometryList[index].rootentries);
/* FIX_ME: FATSz16 for other floppy disk sizes? */
if(disksize == 320) {
host_writew(&sbuf[FATSz16],1);
sect_per_fat = 1;
}
else if(disksize == 360) {
host_writew(&sbuf[FATSz16],2);
sect_per_fat = 2;
}
else if(disksize == 720) {
host_writew(&sbuf[FATSz16],3);
sect_per_fat = 3;
}
else if(disksize == 1200) {
host_writew(&sbuf[FATSz16],7);
sect_per_fat = 7;
}
break;
}
index++;
}
}
// BIOS drive
sbuf[DrvNum] = (mediadesc == 0xF8) ? 0x80 : 0x00;
// file system type
if (FAT >= 16) sprintf((char*)&sbuf[0x36],"FAT16 ");
else sprintf((char*)&sbuf[0x36],"FAT12 ");
if (FAT >= 16) sprintf((char*)&sbuf[FilSysType],"FAT16 ");
else sprintf((char*)&sbuf[FilSysType],"FAT12 ");
}
// boot sector signature
host_writew(&sbuf[0x1fe],0xAA55);
host_writew(&sbuf[BootSign],0xAA55);

// if anything should try to boot this partition, add code to print an error message instead of
// letting the CPU run wild through not executable code.
if (FAT >= 32) {
// the code expects to load a string from a fixed address.
// we're relocating it to make room for FAT32 structures so some patching is required.
memcpy(sbuf+0x5A,this_is_not_a_bootable_partition+0x3E,0x1FE - 0x5A);
memcpy(sbuf+BootCode32,this_is_not_a_bootable_partition+0x3E,BootSign - BootCode32);
host_writew(sbuf+0x5D,0x7C77); // 0x7C5D: MOV SI,<stringaddr> we are patching the <stringaddr>
}
else {
memcpy(sbuf+0x3E,this_is_not_a_bootable_partition+0x3E,0x1FE - 0x3E);
memcpy(sbuf+BootCode,this_is_not_a_bootable_partition+0x3E,BootSign - BootCode);
}

// write the boot sector
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